In recent years, a synthetic quartz glass substrate used for semiconductor-related electronic materials for cutting-edge application is required to meet stringent standards for qualities such as defect density, defect size, surface roughness, and flatness, due to miniaturization of a pattern by photolithography. Above all, further quality is required for defects on the synthetic quartz glass substrate as integrated circuits become finer and magnetic media have higher capacity.
In view of this, a polishing agent used for polishing a synthetic quartz glass substrate is required to improve the quality of the synthetic quartz glass substrate after polishing. More specifically, it is strongly required that the polished synthetic quartz glass substrate has small surface roughness and that the polished synthetic quartz glass substrate has few surface defects such as a scratch on the surface. Furthermore, in view of productivity improvement, it is also required to improve the polishing rate of the synthetic quartz glass substrate.
Conventionally, in general, a polishing agent containing silica (SiO2)-based abrasive grains has been investigated to be used as the polishing agent for a synthetic quartz glass substrate. Silica-based slurry is produced by subjecting silica particles to grain growth through thermal decomposition of silicon tetrachloride and adjusting pH with an alkaline solution containing no alkali metal such as ammonia.
For example, Patent Document 1 describes that defects can be reduced by using high-purity colloidal silica around neutrality. However, considering the isoelectric point of colloidal silica, colloidal silica is unstable around neutrality, and there is fear that the colloidal silica abrasive grains fluctuate in particle size distribution during polishing, and cannot be stably used. Thus, the silica-based polishing agent is difficult to be circulated and repeatedly used for polishing, and is necessarily disposed at one-time use, resulting in an economically unfavorable problem.
Moreover, Patent Document 2 describes that defects can be reduced by using a polishing agent containing an acid and colloidal silica having an average primary particle size of 60 nm or less. However, this polishing agent is insufficient to satisfy current requirements of reducing defects, and requires developing to reduce more defects.
On the other hand, ceria (CeO2) particles, which have lower hardness than silica particles and alumina particles, are difficult to cause defects such as a scratch on the surface of the synthetic quartz glass substrate after polishing. Therefore, a polishing agent using the ceria particles as polishing abrasive grains is effective in reducing defects. Moreover, the ceria particles are known as a strongly oxidizing agent and have chemically active characteristics. Thus, the ceria-based polishing agent is effectively used for polishing an inorganic insulator such as glass.
However, the ceria-based polishing agent typically uses dry ceria particles. The dry ceria particles have irregular crystal shapes. Thus, when the dry ceria is used in a polishing agent, defects such as a scratch are easily generated on the surface of the synthetic quartz glass substrate. On the other hand, wet ceria particles have a more stable polyhedral structure than the dry ceria particles. This structure allows defects such as a scratch to be significantly improved, compared with the conventional dry ceria particles.